The invention relates to the preparation of polyhydroxycarboxylic acids by the selective oxidation of di-, tri-, oligo- and polysaccharides in an alkaline medium using an oxygenous gas in the presence of palladium on a carrier as catalyst and bismuth as promoter.
Such a process is known from U.S. Pat. No. 4,985,553, and a further working-out of the oxidation of lactose can be found in an article by H. Hendriks et al. in Carbohydrate Research 204 (1990), 121-129.
A drawback to the process described in said publications is that although the percentage of polysaccharides which can be converted in this manner is over 90%, notably in the oxidation of lactose it fails to advance beyond 95%. In Example V of the aforementioned patent publication a conversion rate of 91% is listed for the oxidation of lactose. Such a mixture of polysaccharide or aldose and salt of the corresponding carboxylic acid is hard to purify on a commercial scale. Moreover, the product was found to be too badly contaminated with residual catalyst. A further drawback.sup.to the process claimed in the U.S. patent is the laborious catalyst preparation. It involves mixing a solution of a bismuth compound, with vigorous stirring, with an aqueous suspension of the palladium deposited onto a carrier. The whole is made alkaline by adding a base, followed by reduction of the bismuth compound with formalin, sodium formate, sodium boron hydride, hypophosphorous acid, hydrazine, glucose, or other reducing sugars. The catalyst thus reduced is filtered, washed, and dried.